Category Archives: observation

2nd harmonic oscillation of a rope full with balloons — and announcing a summer break

On one of my very last days in my old job, I walked across campus and noticed this:

I had to, of course, alert all my nearby friends! Especially since, on this engineering campus, apparently nobody else stopped to take a quick video. Seriously, what’s wrong with the world?

I obviously had to come back at different wind conditions to get a better dataset, which I proudly present in the movie below:

It felt a bit like the university was waving me goodbye, and a part of my life was ending. Which is not a bad thing necessarily, but before I plunge head-first into my upcoming research cruise and then the new job and all the other small or not-so-small commitments that I’ve made, I am going to take it slow(er), take a break and reevaluate my priorities. And for that I will be taking a summer break from regular blogging, too, so you might not hear from me for a bit.

I’ve had this song from the 2007 Hitchhikers Guide to the Galaxy movie stuck in my head for days now. So here we go:

So long, and thanks for all the fish!

Shadows of condensation trails (aka “cirrus aviaticus”)

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It wouldn’t be surprising at all to hear that if a cloud moves in front of the sun, a shadow appears on the ground. But I recently observed pretty much the same thing and found it quite fascinating.

I was looking out of a plane window and saw long dark lines in the clouds below me. For quite some time I tried to come up with processes that might cause that. The line didn’t look like a lee wave of some very high tower. It also didn’t look like it was mirroring a channel or autobahn below. So what could it be?

That became quite obvious when I looked up (which I had tried to avoid before, because the spinning blades of the propeller were a bit unnerving): the dark line was the shadow of a plane’s condensation trail!

Not that surprising after all, but it took me surprisingly long to figure it out.

And it does look pretty cool, doesn’t it?

Front-spotting in muddy waters

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On a recent flight from Hamburg to London City Airport, I ended up on one of the tiniest planes I’ve ever been on. Which meant that we flew super low, I took tons of pictures out of a not-very-clean window, and all my pictures have at least one propeller blade in them.

But look at what we saw!

For example in the picture below, a plume of muddy water coming from some canal into a river (and I should probably know where this is, but I have no idea. Somewhere between Hamburg and London?). I’m not sure whether the inflowing water itself was muddy to begin with, but I would guess that it is stirring up mud from the bottom of the river since it seems to be low tide and the inflowing water is maybe moving a lot faster than the water in the river itself?

Closer to England we flew across this wind farm, where turbines have mud stripes in their lee. Also pretty interesting. Maybe they change direction with tides?

And then coming to the mouth of the River Thames, there is quite a clear front between outflow and muddy North Sea water.

Going upstream on the River Thames, boats stir up a lot of mud!

So you can clearly see where they went for a pretty long time.

On this flight, I sat next to a professional photographer who rolled his eyes at me taking pictures pretty much non-stop. And yes, they might not be the best quality. But at least you see what I saw, right?

Frost flowers on ice cream: When you start thinking about phenomena and something really annoying, all of a sudden, becomes really cool.

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Frost flowers on ice cream. You must have seen them before: They sometimes occur when you’ve had some ice cream, put the left-overs back in the freezer, and take them out again. And there you have it: Water-ice crystals all over your lovely ice cream! Completely annoying because, obviously, they only taste like water and mess up your whole ice cream experience (or is that only me)?

You know I’m kinda fascinated with ice crystals on frozen blended strawberries, but last time I had some, there weren’t only crystalline structures, but there was frost on it:

Frost occurs when water vapour freezes without going through the liquid phase. Look at the awesome crystals!

Once I started thinking about the process that formed the ice and realised that those were actually frost and not just ordinary ice crystals, they all of a sudden stopped being annoying and instead became something that I kinda look forward to finding when I open a tub of my frozen blended strawberries. Because the structures are different every time, and really really pretty! And also how awesome is it to know that those ice crystals formed from water that wasn’t even liquid? Yes, this is the kind of stuff that makes me happy! :-)

Another wave hunt expedition: Learning to discover ocean physics wherever you go

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One of my favourite topics right now: Learning to “see” ocean physics wherever you go. For example here: A visit to my goddaughter in Schleswig, and this time we are practicing all she and her mom read about in MY BOOK (and if you have good ideas for a title for that book, please let me know!). So today I’m showing you pictures of phenomena similar to those in my book, but discovered on this recent visit.

For example diffraction when waves pass this pier:

In the image below, I’m showing what I mean: Waves coming in from the right have straight crests (red). As they pass the pier, they get diffracted and bent around (green).

In this spot, this phenomenon can be seen on most days. I wrote about it before, but I have more pictures from previous visits, where the same thing happens in the opposite direction, too: Waves propagating in from the left and being bent around the pier to the right.

Or we can see other wave crests, meeting a rock that breaks the water’s surface.

Those waves (shown in red in the image below) get reflected from the rock, and circular waves radiate away from the rock (green).

A similar thing can also be observed from a flag moored out in the water:

This time, incoming waves are green and the circular waves radiating off the flag are red.

And we also got to see awesome criss-crossing again, albeit in a different spot:

Here we have the red wave crests coming in, and the green reflections.

If we look at it from a little more distance, we can also see another phenomenon: The wave crests are refracted towards the shallower shore:

Again the red crests are the original, incoming ones, and the green ones are the reflection:

And then finally, let’s look at duckies again. And on waves being created by wind:

Below you see the direction of the wind (white): One side of this little channel is shaded from the wind, so hardly any ripples there. But then on the other side, we clearly see ripples and small waves. And we see the wake the ducky made!

And one last picture: Which direction does this little channel flow in?

Yep. From the left to the right!

If you enjoy discovering this kind of stuff on your walks, or know someone who enjoys it, or want someone to learn to enjoy it, you might want to consider checking out my book. In my book, I show many pictures like those above, but I actually explain what is shown in the pictures rather than assuming (like I do on this blog) that my readers are oceanographers anyway… :-)

Learning about tides from art moored in a river

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Disclaimer: This post might well be called “fun with tides” similar to Sheldon Cooper’s “fun with flags” — it is super nerdy, but at least I am having fun!

There is some really cool art around Hamburg, and the one I want to talk about today is called “four men on buoys” by Stephan Balkenhol: Four wooden statues of approximately live-sized men, standing on little floats, moored in four different spots all over Hamburg. One of them happens to be on the Elbe river, visible when you cross the bridge from where I work over to the city center. You ca see the scene below: The train going across the bridge, and the guy (in the white shirt) standing on the river.

What you can sort of see in the picture above from the yellow buoy being tilted to the right: There is quite a strong current in that river. And what you can’t see in the picture, but will find out below: It’s a tidal current, hence its direction reverses regularly.

You can guess what that means for anything moored in the river: Yes, it will change its position following the tides!

This is where my nerdy self comes in. Whenever I take the train across that bridge, I try to snap a picture of the guy on the buoy. It is quite a difficult endeavour — the train usually goes pretty fast, and I never know where exactly the guy is going to be (well, I guess I could look at a tide table beforehand, but I’ve never done that) and taking pictures out of a train window is not that easy in itself. But sometimes it works out beautifully to show both the position of the guy and the currents:

The guy on the buoy 3 1/2 hours after high water that day

As you see in the image below, the wake is in the direction towards the viewer. This means that the water is flowing towards the viewer, i.e. downstream. You can see that the current is fairly strong because the wake is very pronounced (“very” at least relative to some other pictures you’ll see later).

For this post, I checked my phone and found a collection of 16 pictures of that guy. So clearly I had to see when they were taken relative to the time of high water that day. In the image below, each tick marks the time of one of my pictures relative to zero, the time of the nearest high water.

The guy on the buoy, plus an eye-balled plot of my data points. 0==high water. This picture was taken 1 1/2 hours after high water that day.

As you can see, I seem to be on the train more when it’s close to high water than close to low water. Funny!

Now, when I show you all my 16 data points, let’s remember that we are now only looking at time before/after high water. We are neglecting important things like where exactly the picture was taken from (I’m excited to catch the guy on the buoy at all from a fast train!) or where we are in the spring / neap cycle. Plus the different times of day when the pictures were taken and the different weather conditions make comparison hard. Yet, it’s fun to see how the strength and even direction of the current (which you can see by looking at the wake and the position of the guy relative to the bridge) is changing!*

Before I show you the pictures, a CALL TO ACTION: If you happen to be on that train, snap a picture and send it to me! I’ll happily compile a better series with more data points! I’ll continue taking pictures, too, that’s for sure! :-) Imagine how you could use this kind of data in teaching! If I were to teach a class on tides at this university, I would have students collect this type of data and use it to say something about the tides on Elbe river. If there is enough data (which should be easy enough to get with many students commuting across this bridge every day), I am sure one could learn a lot from this case study! And working with data students collect themselves is always more fun than looking at some data set in a text book anyway. Plus how much more exciting would commuting get for those students once they start observing in this way, and starting to think about water, instead of just being bored on the train? There are actually a couple more times where you see the river quite well on the train journey between university and the city centre, so there might be many more case studies easily done if only people started looking for them…

And here are all 16 pictures, in the order going from low water to high water to low water. The caption includes the time before/after high water. Enjoy!

Guy on buoy. Picture taken 4h 50min before high water.

Guy on buoy. Picture taken 4h 02min before high water.

Guy on buoy. Picture taken 2h 34min before high water.

Guy on buoy. Picture taken 1h 14min before high water.

Guy on buoy. Picture taken 1h 06min before high water.

Guy on buoy. Picture taken 1h 02min before high water.

Guy on buoy. Picture taken 0h 27min before high water.

Guy on buoy. Picture taken 0h 26min before high water.

Guy on buoy. Picture taken 0h 25min before high water.

Guy on buoy. Picture taken 0h 12 min past high water.

Guy on buoy. Picture taken 0h 48min past high water

Guy on buoy. Picture taken 1h 17min past high water.

Guy on buoy. Picture taken 1h 38min past high water.

Guy on buoy. Picture taken 1h 48min past high water.

Guy on buoy. Picture taken 3h 32min past high water.

Guy on buoy. Picture taken 6h 05min past high water.

*Btw, sometimes you see that my mapping is clearly not right (for example, when the wake is in the direction away from the viewer, we cannot be past high water already, since the current is clearly still going up the river, so the tide hasn’t turned yet). These errors might be due to me not taking enough care when looking up the tidal data (yep.) or the tide tables that were used not accounting for factors that might have influenced the tides other than the classical tidal components, like for example wind conditions. I could, of course, go back and look at actual data and/or double-check, but I am happy with what I can see from the data already. If you are not, please knock yourself out and I’d be happy to host your guest post with corrections of my post! :-)

Observe a fresh water layer (with your eyes, not a CTD!)

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Sometimes you actually see fresh water layers (see with your eyes, not a CTD or some other instrument) floating on top of denser waters, not only in your kitchen and with the help of dye, but for real. In this case, you see the layers because the shadow of a pole appears twice — once on the surface itself, and once on the interface between the layers.

See below: Shadow on the surface between the red lines, on the interface between green lines, and the reflection on the surface between blue lines.

I took these pictures on a trip to Husum with my sister and her family.

Foam stripes parallel to the coast.

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On my way to Heligoland the other day I noticed a phenomenon that I found really intriguing and that I should probably be able to explain. I first saw it on the screen of the boat’s web cams when we were about to leave the port of Hamburg. Unfortunately I could, at that point, only take a photo of the screen (but see how I excited I was to actually take a photo of the screen? ;-)).

Can you spot what I mean? Yes, that foam stripe running pretty much parallel to the pier! The place where it bends is right in front of our boat, which you see the railing off as that white stuff at the bottom of the screen).

But then, on Heligoland, I saw it again and became even more intrigued. Right in front of the place I stayed at, you could see it looking down the coast to your left…

…and to your right!

Here we can also see the stripe bending at some point, but here again the bend coincides with a change in the coast line. Similarly to what we saw in the port of Hamburg above, the stripe stays at more or less the same distance from the coast, so it is parallel while the coast is straight, and bends out when there are obstacles (like the catamaran above or the rocks below).

So how do we interpret the whole thing? I am not quite sure. I seem to have a very vague recollection that it should have something to do with half a wave length of the dominant wave, and foam collecting in a node point. And that makes sense intuitively. Except that I have several (ha! understatement of the month) minutes of video footage of the above, and I cannot for the life of me spot anything that would explain the stripe. If it is a node point, it is a very well-disguised one and I am surprised the foam can find it!

But there must be something different about that location than about all the other places closer to or further away from the coast. Any ideas, anyone?

Looking at how waves are bent

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As soon as waves run into water that becomes shallow enough for the wave to “feel” it, the waves start changing how fast they propagate. And depending on the shape of the topography, this often means that the waves change direction. This is called refraction and we can see it on many many different scales.

On a large scale, we see the wind waves coming down a fjord with long, straight crests, therefore not influenced by the sea floor yet . But when those waves come close to the shore, they start feeling the sea floor and react to that by being bent towards the shallower depth:

Below, I have drawn the wave crests I want you to focus on:

Straight crests coming in from the left, and then bent crests running into the mouth of the little bay to the right. Seeing it put like this, this might actually also be diffraction if we think of the mouth of the bay as of the widening behind a slit that the waves came through. Oh well.

But the same thing can also be seen on smaller scale, for example in the bay shown in the picture below. You can see some of the topography through the water, and you can infer some more: Wherever sea weed breaks the surface, water around it is probably shallower than water a little further away. And when you look closely, you can again see the waves reacting to the topography.

Similar thing again below:

Btw, this little bay is the one you see on the right of the picture on top of this blog post!

And it is not only physics of water that make this place super interesting: repeat after me, Ib: Anemones! :-)

Although it is pretty cool to spot more wave processes, like for example the wind shade — no waves phenomenon.

Again on large and small scales:

And you see the interference of waves coming round either side of those tiny “islands”.

Or just look at the beautiful blue water, the fjord, the snowy mountains in the back…

Or at something completely different: The salt rings that remain when rock pools fall completely dry.

Anyway, I love it here. Thanks, Elin, for having me again!

Adventures in Oceanography and Teaching

Dr. Mirjam S. Glessmer

Category Archives: observation

2nd harmonic oscillation of a rope full with balloons — and announcing a summer break

Shadows of condensation trails (aka “cirrus aviaticus”)

Front-spotting in muddy waters

Another wave hunt expedition: Learning to discover ocean physics wherever you go

Learning about tides from art moored in a river

Observe a fresh water layer (with your eyes, not a CTD!)

Foam stripes parallel to the coast.

Looking at how waves are bent